1. Field of the Invention
The present invention relates to an antenna, more particularly to a coaxial cable fed inverted-L antenna for a simple and easy installation.
2. Description of the Related Art
In general, the dipole antenna 2 usually used in a traditional wireless communication device 1 (as shown in FIG. 1) is an coaxial sleeve antenna (as shown in FIG. 2); such dipole antenna 2 comprises a coaxial cable 10, and such coaxial cable 10 comprises an internal conductor 14 (or a symmetric axis); an external conductor 16 (or mask or ground); an insulated dielectric material 17 disposed between the internal conductor 14 and the external conductor 16 for the isolation such that the internal conductor 14 and the external conductor 16 constitute a so-called concentric conductor in electromagnetism. Further, the external edge of the coaxial cable 10 is wrapped by an insulating external skin 19 with one end coupled to a control circuit (not shown in the figure) of the wireless communication device 1, and the other end has a metal sleeve 18, such that the metal sleeve 18 is coaxial with the external conductor 16, and only the upper end of the metal sleeve 18 is connected to the external conductor 16, and the rest of the metal sleeve 18 is separated from the area of the external conductor 16 by the insulating external skin 19, instead of contacting with the external conductor 16. The internal conductor 14 is extended to an appropriate distance from another end of the coaxial cable 10, and the length of such distance is approximately equal to the length of the metal sleeve 18, but both of them are slightly shorter than a quarter of the wavelength of the operating frequency (¼ λ; where λ is the wavelength of the operating frequency), so that another coaxial conductor is formed between the metal sleeve 18 and the external conductor 16 to prevent the radioactive interference produced by the leaked current on the outer side of the external conductor 16, which constitutes a balance-unbalance (balun) converter in order to produce the expected antenna radiation by the coaxial cable sleeve antenna.
In general, an all-directional radiation filed antenna must be installed to a mobile or portable wireless communication device such as the present commonly-used mobile phone, so that such wireless communication device can maintain a 360 degrees azimuth communication. The aforementioned dipole antenna is the antenna commonly installed to such wireless communication device, and such dipole antenna is generally installed to receive or send high-frequency (HF), very high-frequency (VHF), and ultra high-frequency (UHF) signal or a wireless communication device, and its basic structure mainly uses a metal sleeve 18 on the coaxial cable sleeve antenna to design a balun converter. Further, to enhance the performance of the antenna and maintain the all-directional radiation field, a collinear structure is used to design such coaxial sleeve antenna.
Since the IEEE 802.11 wireless local area network protocol established in 1997, such protocol not only provides unprecedented functions on wireless communication, but also offers a solution for the mutual communication between different branded wireless products. Therefore such protocol opens up a new mileage to the development of wireless communication, and creates the demand of mobile communication products in the market, such that the development of wireless communication becomes much faster. Thus, in recent years, many wireless communication product designers and manufacturers have been expecting an antenna with a simple structure, easy installation, and low cost while developing a wireless communication device to receive or transmit the high-frequency (HF), very high-frequency (VHF), and ultra high-frequency (UHF) signals in order to effective lower the cost of the antenna which is used in such wireless communication products.
However, the present common high-frequency antennas sold in the market also includes a printed antenna production technology, design, and manufacture equivalent to the dipole antenna 2 in addition to the traditional coaxial cable sleeve antenna. Please refer to FIG. 3. Such the dipole antenna 2 comprises a dielectric substrate 20 in the shape of a board, a first printed circuit 22 and a second printed circuit 24 respectively printed on the front and back sides of the dielectric substrate 20, wherein the first printed circuit 22 printed in the front side acts as the signal transmission line with one end acting as the signal fed end 21 to connect to a control circuit (not shown in the figure) of a wireless communication device through an internal conductor 31 (or symmetric axis) of a coaxial cable 30, and another end of the first printed circuit 22 has a radiating member 25 extended to the corresponding side with an appropriate distance, and the length of such radiating member 25 is slightly shorter than a quarter of the wavelength of the operating frequency (¼ λ; where λ is the wavelength of the operating frequency). Further, the second printed circuit 24 on the back side of the dielectric substrate 20 being printed substantially in M-shape on the position corresponding to the signal feed end 21 at the front side, and the middle of the printed circuit 241 on the second printed circuit 24 connects to a ground end (not shown in the figure) on a control circuit of the wireless communication device through the external conductor 32 (or mask or ground) of the coaxial cable 30, and a concentric conductor is formed between the printed circuit 242 on both sides and the printed circuit 241 in the middle to prevent the radiation interference produced by the current leaked from the printed circuit 242 on both sides to constitute a balun converter, so that the antenna can produce the expected radiation effect as that of the aforementioned coaxial cable sleeve antenna.
Generally speaking, regardless of using the printed technology to produce the printed antenna or the traditional coaxial cable sleeve antenna, the features can meet the requirement, but the size of such traditional antenna is too big, or the structure is too complicated. For example, the traditional coaxial cable sleeve cable requires soldering the upper end of the metal sleeve with the external conductor of the coaxial cable, but the printed antenna requires an additional printed circuit board for its design and production. Furthermore, the coaxial cable and the printed circuit board of the antenna is soldered manually, not just increasing the production, manufacturing, and assembling costs, but also wasting the unnecessary installation space. In addition, since the microwave frequency has a shorter wavelength, therefore, the variation is relatively higher during the soldering process of manufacturing such dipole antenna, which will cause a low yield rate.